A fuel cell converts chemical energy into electrical energy and some thermal energy by means of a chemical reaction between a fuel (e.g., a hydrogen-containing fluid) and an oxidant (e.g., oxygen). A proton exchange membrane (PEM) fuel cell uses hydrogen or hydrogen-rich reformed gases as the fuel, a direct-methanol fuel cell (DMFC) uses methanol solution as the fuel, and a direct ethanol fuel cell (DEFC) uses ethanol solution as the fuel, etc. These types of fuel cells that require utilization of a PEM are collectively referred to as PEM-type fuel cells. As compared to other energy sources, fuel cells provide advantages that include low pollution, high efficiency, high energy density and simple fuel recharge. Fuel cells can be used in electrochemical engines, portable power supplies for various microelectronic and communication devices, standby power supply facilities, power generating systems, etc. Further, fuel cells utilize renewable resources and provide an alternative to burning fossil fuels to generate power.
A PEM-type fuel cell is typically composed of a seven-layered structure, including (a) a central PEM layer for proton transport; (b) two electro-catalyst layers on the two opposite sides of the electrolyte membrane; (c) two gas diffusion electrodes (GDLs) or backing layers stacked on the corresponding electro-catalyst layers (each GDL comprising porous carbon paper or cloth through which reactants and reaction products diffuse in and out of the cell); and (d) two flow field plates or bi-polar plates stacked on the GDLs. The flow field plates are made of carbon, metal, or conducting composite materials, which also serve as current collectors. Gas-guiding channels are defined on a GDL facing a flow field plate, or on a flow field plate surface facing a GDL. Reactants and reaction products (e.g., water) are guided to flow into or out of the cell through the flow field plates. The configuration mentioned above forms a basic fuel cell unit. Conventionally, a fuel cell stack comprises a number of basic fuel cell units that are electrically connected in series to provide a desired output voltage. If desired, cooling plates and humidifying plates may be added to assist in the operation of a fuel cell stack.
Several of the above-described seven (7) layers may be integrated into a compact assembly, e.g., a membrane-electrode assembly (MEA). An MEA typically includes a polymer electrolyte membrane bonded between two electrodes—an anode and a cathode. Typically, there exists an electro-catalyst layer between the membrane and the anode, and another electro-catalyst layer between the membrane and the cathode. Hence, an MEA is typically a five-layer structure. Most typically, the two catalyst layers are coated onto the two opposing surfaces of a membrane to form a catalyst-coated membrane (CCM). The CCM is then pressed between a carbon paper layer (the anode) and another carbon paper layer (the cathode) to form an MEA. Alternatively, a catalyst layer is deposited onto one primary surface of a carbon paper before this surface is pressed against one surface of the membrane. Commonly used electro-catalysts include noble metals (e.g., Pt), rare-earth metals (e.g., Ru), and their alloys. Known processes for fabricating high performance MEAs involve painting, spraying, screen-printing and hot-bonding catalyst layers onto the electrolyte membrane and/or the electrodes.
Hydrogen ion or proton transport through the PEM layer in a PEM-type fuel cell requires presence of water molecules within the membrane such as poly (perfluoro sulfonic acid) or PFSA (such as du Pont's Nafion®), its derivative, copolymer, or mixture. Consequently, it is critical to maintain adequate membrane hydration in order for the fuel cell to function properly. In addition to maintaining adequate ionic conductivity for proton transport, uniform membrane hydration serves to prevent localized drying, or hot spots, that could result from higher localized resistance. In general, dehydration may impede performance, increase resistive power losses, and degrade the integrity of the membrane.
In conventional fuel cells, membrane hydration is achieved by humidifying the fuel (e.g. hydrogen gas) and oxidant gases (e.g., oxygen or air) prior to their introduction into the fuel cell. One commonly used method for pre-humidifying fuel cell gas streams is to employ membrane-based humidifiers (e.g., Reid, U.S. Pat. No. 6,403,249, Jun. 11, 2002). In these situations, reactant moisture is added by allowing the respective gases to flow on one side of a water vapor exchange membrane while directing deionized water to flow on the opposite side of the membrane. Water is transported across the membrane to humidify the fuel and oxidant gases. Another known technique for pre-humidifying the reactant gas streams entails exposing the gases directly to water in an evaporation chamber to permit the gas to absorb evaporated water. Alternatively, humidification may be achieved by directly injecting or aspirating water into the respective gas streams before introducing them into the fuel cell.
Generally, pre-humidification is undesirable because it requires auxiliary fuel cell components, increasing the relative complexity of a fuel cell system. For instance, pre-humidification generally requires dedicated components for storing and transporting water. Auxiliary water storage and transport components reduce operating efficiency and add to the overall weight and cost of the system. Additional weight is an undesirable feature for a fuel cell if the cell is to be used in a portable microelectronic device such as a mobile phone or a personal data assistant (PDA). Additional components may also present system reliability issues. For example, where fuel cells are operated in sub-freezing conditions, water solidification can result in the weakening of mechanical components.
Wynne, et al. (U.S. Pat. No. 6,207,312, Mar. 27, 2001) disclosed a self-humidifying fuel cell that made use of the reaction product (water) as a source of PEM moisture, avoiding the use of auxiliary components. However, this fuel cell requires the design and construction of complex flow field channels in the gas diffusion electrodes or the flow field plates.
An interesting approach to maintaining PEM hydration is to add a filler as a moisture retainer. For instance Yuh's utilized superacids as both proton conductors and moisture retainers in a Nafion membrane (C. Y. Yuh, “R&D on an ultra thin composite membrane for high temperature operation in PEMFC,” 2003 Hydrogen and Fuel Cell Merit Review, Berkely, Calif., May 19-22, 2003). Stonehart, et al. incorporated silica as a moisture retainer in an ion-exchange resin (“Polymer solid electrolyte composition and electrochemical cell using the composition,” U.S. Pat. No. 5,523,181 (Jun. 4, 1996)). Watanabe, et al. added a metal catalyst in a polymer electrolyte to catalyze the chemical reaction between hydrogen and oxygen molecules that diffuse into the electrolyte membrane to produce water therein (“Solid polymer electrolyte composition,” U.S. Pat. No. 5,766,787 (Jun. 16, 1998)). Watanabe, et al. further proposed to add a metal oxide to help retain the water produced. However, most of the metal oxides were not very effective water retainers or water captors. Hence, metal oxides in a PEM did not further improve the performance of a fuel cell operated at a temperature higher than 80° C. or at a humidity level lower than 50% RH. In one of our earlier inventions (B. Z. Jang, “Self-Moisturizing Proton Exchange Membrane, Membrane Electrode Assembly and Fuel Cell,” U.S. Patent Pending (Ser. No. 10/657,038) Sep. 8, 2003), a deliquescent material was added to a PEM to significantly improve the membrane's ability to capture and retain moisture. None of the aforementioned approaches made use of a judicious combination of a metal catalyst and a deliquescent material dispersed in a polymer matrix to form a self-humidifying membrane for fuel cell applications.
Currently, PFSA polymers and their close derivatives dominate the membrane market for PEM fuel cells. These polymers can only be used in fuel cells that operate at relatively low temperatures (<80° C.). Fuel cells that operate at higher temperatures (>120° C. or even >150° C.) offer several advantages: increased catalytic activity (fast electrode kinetics), higher tolerance to fuel impurities (simplified reformer-purification system resulting in reduced cost, weight, volume, and response time), simplified water management (simplified stack construction and operation), and increased value of heat recovery. The need or desire to operate fuel cells at higher temperatures presents difficult new challenges for the PEM polymers. This difficulty stems primarily from the decrease in water content of the polymer electrolytes in the desired temperature range. Specifically, when a fuel cell is operated at a higher temperature, the PEM therein tends to get severely dehydrated, thereby significantly degrading the proton conductivity. Although thermally stable polymers such as poly (ether ether ketone) or PEEK have been sulfonated to produce proton-conducting membranes, their reliability and stability in a fuel cell operating in high temperature and low humidity conditions have remained questionable. Clearly, there is an urgent need for a PEM that can operate at a temperature higher than 120° C. in a low humidity environment (25-50% RH).